Nordex: Twenty-five years of turbine technology

1 September 2010
by Eize de Vries

GERMANY: German wind turbine manufacturer Nordex last month celebrated its 25th anniversary at its main production plant at the German port of Rostock. Executives tell Eize de Vries the company is putting technology and industrial quality at the centre of future developments.

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Back in 1985, long before renewables were a focus for the energy industry, brothers Carsten and Jens Pedersen acted on a vision that wind would become an economically viable source of electricity. They founded a turbine firm called Nordex in Give, Denmark.

In the 25 years since, the company has installed more than 4,000 wind turbines comprising nearly 6GW and has listed on the Frankfurt stock exchange. In addition to its German production facilities, Nordex has nacelle and rotor blade factories in China, and plans to begin production at a US nacelle plant next month.

Over the years, the firm has repeatedly pushed the envelope of wind power technology in terms of power rating and, today, is among the world's leading manufacturers of megawatt-scale machines. As competition from newcomers, particularly the Chinese, grows tougher, Nordex has fallen from the world's top ten global companies. But the company has stayed in the hunt through continued technological advances and forays into offshore wind development aimed at giving it a stake in the burgeoning market.

Experience has been key. The 250kW N27 wind turbine the company introduced only two years after its inception was, at the time, one of the world's largest. In 1991, the company formed a German unit, Nordex Energieanlagen, in the small town of Rinteln and soon launched its first German factory in Rerik on the Baltic Sea. This local Nordex team anticipated huge German demand for wind. In an unusual development in 1993, the Nordex Energieanlagen team seized the moment and independently leapt into the 800-1,000kW class of wind turbines - sidestepping what they perceived as the hesitancy of the Pedersen brothers to pursue turbines exceeding 500kW capacity.

Commercial first

The team's gambit succeeded. A German federal state grant in 1995 helped it produce the 800kW N52/800 prototype. Scaled up to 1MW before long, the N54/1000 became the world's first commercial megawatt-class turbine to be manufactured in series. In 1996, German component manufacturer Balcke-Durr saw promise in Nordex's business strategy and acquired the company.

Fast forward to 2000, when the pitch-controlled variable-speed N80/2500 prototype with a 2.5MW power rating and an 80-metre rotor blade was installed. It was the world's first 2.5MW-class commercial wind turbine. To this day, 2.5MW turbines serve as Nordex's main product offerings.

Nordex chief executive officer Thomas Richterich describes a corporate strategy of providing customers with reliable turbines that reduce costs over operational lifespans. "Achieving these goals requires continuous innovation, advanced industrial processes and methods ranging from research and development (R&D) to manufacture and installation upkeep," he says. "Today, most of my colleagues on the management board originate from established industries, including the automotive sector. Our aim is to learn from best practices in every industry."

Customers appear to be noticing. Nordex suffered a 31% decline in total revenues in the first quarter, compared with the same period in 2009, and a 9.1% fall in gross profits over the same period. Its share price fell 24% between January and July. But it experienced a strong second-quarter surge in demand with an order intake in April alone that was 32% higher than that of the entire first quarter. Nordex forecasts single-digit sales growth throughout 2010.

Improved specifications

Richterich cites two examples of how Nordex will try to keep pace with - and, if possible, stay ahead of - the technology curve. One is a robotic system for rotor-blade finishing; another is a recent shift to continuous-flow production at the nacelle plant in Rostock (see box, page 114).

This year, the company introduced the third-generation 2.5MW Nordex Gamma series, which builds on more than 1,300 previous-generation turbines put into operation over a decade. The Gamma platform adds the N100/2500 turbine, with a 100-metre rotor diameter, to the existing N80/2500 and N90/2500 models with 80- and 90-metre rotors, respectively.

Richterich explains that the N90/2500 and N100/2500 share about 95% of the same components, a characteristic known as modularity. A modular turbine enables the use of a similar platform and many identical components to cut costs. It also allows wind firms to keep smaller stocks of spare parts. A similar strategy has been successfully employed in the car industry.

Richterich rattles off a list of other specifications. Nordex has increased average energy yield from the N80 and N90 by 5% compared to earlier models, he says. He adds that the N100 yields an average 20% more energy at low and medium wind speeds than the N90. A new cast design for the main chassis has resulted in a three-tonne reduction in weight, reducing materials costs for Nordex and reducing dynamic loads across the entire system, from rotor to foundation. Nacelles are equipped with climate-control technology aimed at eliminating ambient air from the nacelle. At coastal locations, air can corrode turbine interiors, sometimes resulting in premature failures. Nordex has also taken many steps to improve working conditions inside nacelles for service personnel.

Finally, the company has sought to reduce customer costs from transport and cranes by setting a 50-tonne weight limit for individual turbine components. Other suppliers also aim at similar optimising measures.

Nordex's broader product range includes a 1.5MW unit manufactured in China for the local and broader Asia Pacific markets. It features an 82-metre rotor, stepped up from the previous 77-metre diameter. Richterich says, "For onshore wind markets, we will in future introduce even larger rotors aimed at further reducing costs of energy. This means a new rotor of 110 metres for our 2.5MW series and 88 metres for the 1.5MW series."

Design principles

Chief technology officer Eberhard Voss explains why the company is placing its bets on fast-speed drive systems with gearboxes even as many competitors move towards direct-drive turbines in which rotors connect directly to slow-speed generators. He believes critics of gearboxes misunderstand the most common causes of breakdown, blaming gearbox technology unfairly. "Many issues linked to wind turbine gearboxes can be traced back to design flaws and production failures," he says.

"Typical examples of the first category are a mismatch between calculated loads and actual loads, a poor overall design, and common issues like insufficient cooling and lubrication," Voss adds.

Voss says there have been great strides in gearbox design in recent years. He points to so-called differential gearboxes developed by Germany's Bosch-Rexroth, one of Nordex's main gearbox suppliers. In this modern gearbox design, the highest loads on the drive system, at the input shaft, are split into two parallel load tracks in a 40-60% ratio. These tracks merge at the low-torque gearbox output shaft.

The objective of load splitting is to enhance lifetime and reliability. Voss says Nordex's advanced abilities to simulate turbine loads in the turbine-design phase, as well as to control operational loads inside turbines, are additional factors contributing to enhanced reliability and extended lifetime.

"Until 2015, Nordex will apply three-stage gearboxes in the 2.5MW turbines, which is a decision based upon design, costs as well as operations-and-maintenance-related considerations," says Voss. "One main consideration is crane costs related to individual component mass, which has proven a main cost driver during gearbox or generator exchange."

In another technology trend, several major turbine suppliers are considering a switch from commonly used doubly fed induction generators (DFIGs). These generator/power converter systems can accommodate wind of variable speeds with power-converter systems of only one-third of full rated power to achieve lower system cost. Now there is rising interest in permanent-magnet synchronous generators using full converters.

"We have recently conducted a comparative study on both alternatives, focusing on total systems efficiency, main characteristics including thermal behaviour, costs, reliability and turbine-control flexibility. Based upon these outcomes, we decided to continue using DFIGs in our 1.5MW and 2.5MW turbine series," Voss says.

With the Gamma platform development in its final stages, the Nordex R&D team is concentrating on other product developments, both for onshore and offshore.

As it did with the Gamma series, Nordex again chose a modular approach to its much larger 3-4MW onshore turbine, says Voss. This time, however, the maximum mass for main components was 75 tonnes. "A two-stage medium-speed drive system was the final outcome of our comparative concept review on different alternative solutions," he explains. Key design drivers included maximised reliability, design flexibility and optimised energy costs. Voss adds that for maximum yield performance, the machine will be fitted with a large rotor, offering up to 4,000-square-metres rotor swept area per megawatt of installed power. Nordex plans to install an onshore prototype in the first quarter of 2012.

Gazing offshore

As for development of offshore turbines, Voss says that lessons have been learnt from trying to adapt two onshore 2.5MW N90/2500 turbines for offshore use - one operating off Denmark in Frederikshavn and the second in Rostock harbour. "Initially, our view was that a good onshore wind turbine with some marine modifications also works well offshore," he says. "Today, I am convinced that offshore operation requires a dedicated product."

Before launching into the design process for its offshore turbines, Nordex consulted key offshore wind developers. "The main discussion topics were their views on essential turbine design and operational capabilities, preferred power rating range, investment costs, and operational cost price ranges," says Voss.

Based on these talks, Nordex decided on a highly compact wind turbine with a power rating of 5-6MW. The turbines will use synchronous generators with full converter and either a single-stage geared or direct-drive system. Voss said the firm is in the concept-design phase and an offshore prototype will be erected at an onshore location in the first quarter of 2012.

Earlier this year, Nordex acquired a 40% share in Germany's Arcadis Ost 1 offshore wind project. The aim is to secure fast entry into the rapidly growing wind market segment. The project will comprise about 70 wind turbines with a combined installed capacity of 300MW in a water depth of about 40 metres. With the first Nordex offshore wind turbine installations planned by 2014, that gives it only about two years to complete prototype testing, optimisation and certification. A smooth-running offshore unit must be put together, as well as a seamless supply chain.

This would be a mammoth challenge for any company. Asked whether the combination of two parallel product developments and a tight implementation schedule does not put an enormous strain upon his team, Voss said Nordex is up to the task. "From 2004 onwards we have gradually expanded our R&D department and also brought it fully in line with today's demands," he says. "Perhaps most important, we are a strong team comprising excellent engineers and other specialists proud to be working on these projects and determined to succeed."

THE NEXT STEP - BREAKING NEW GROUND WITH BLADE RESEARCH

As part of its expansion strategy, German wind turbine manufacturer Nordex earlier this year inaugurated a new Rostock-based rotor blade factory, including three halls with a combined area of 35,000 square metres. These are split into blade manufacture, finishing and testing. In the advanced testing facility, blades - mainly prototypes - are subjected to static and dynamic testing.

The facility can handle blades up to 65 metres - considerably longer than the NR50s, which are the largest blades currently made in-house, at about 49 metres.

During static testing, engineers clamp a blade in a winch frame by its root section, then apply downward and side-to-side loads. In dynamic blade testing, an attached oscillating weight pushes and pulls a blade at predetermined sequences in horizontal and vertical directions.

This testing method is still uncommon among turbine suppliers. The dynamic testing enables so-called highly accelerated life testing (Halt), in which a blade's 20-year operational lifespan can be simulated within six months.

Since 2008, Nordex has invested more than EUR100 million in the factory, raising annual production capacity from about 120 blades, with a combined rated capacity of 300MW, to 1,500 blades (1.25GW). These are for the N90/2500 and N100/2500 series. Nordex produces about half of its blades in-house, a rate it plans to maintain.

The company plans to step up advanced industrial processes at its nacelle unit, as well as at the blade manufacturing facility.

Nordex chief operating officer Marc Sielemann believes the wind industry can benefit from applying manufacturing know-how from the aerospace and automotive industries. He cites the recent introduction of continuous-flow production in Nordex's nacelle assembly plant in Rostock.

"Our main focus is to achieve the required efficiency gains, as well as enabling the planned production expansion for the next years," Sielemann says of both the nacelle and the blade factories.

As another example in the blade factory, Nordex introduced a three-shift system over 6.5 days a week. A precisely tuned production process and advanced chemical process are needed to speed up production while maintaining high quality standards.

Nordex cites its blade finishing process as another example of advanced industrial production. A blade is first lowered into a cabin and fixed in place. Doors close around the blade and two robots clean the entire surface of the blade from either side and apply a kind of pre-coating. Two other robots then spray-paint the blade, quickly finishing the job. Finally, this coating is cured by a mobile infrared heating device, which is one of several technologies used to reduce curing process time.